The present invention relates generally to acquiring a signal from a device under test and more particularly to a signal acquisition system having reduced loading of the device under test using a signal acquisition probe with reduced capacitance.
Traditional passive voltage probes 10 generally consist of a resistive-capacitive parallel network 12 at the probe tip 14, shown as RT and CT in FIG. 1, coupled via a resistive center conductor signal cable 16 to compensation circuitry 18 in a compensation box. The compensation circuitry 18 has resistive elements RC1 and RC2 and capacitive element CC. RC1 is in series with the cable 16 and RC2 is in series with variable capacitor CC. The compensation circuitry 18 is coupled to input circuitry 20 of a measurement test instrument 22, such as an oscilloscope, spectrum analyzer, logic analyzer and the like. Generally, the input circuitry 20 of an oscilloscope includes an input resistive-capacitive network 24, shown as RTS and CTS, that is associated with switching input attenuation circuitry (not shown) that provides an input impedance for the oscilloscope of 1 MΩ in parallel with 10 to 20 picofarad (pf) of capacitance. The output of the switching input attenuation circuitry is coupled to the input of a preamplifier 26. The oscilloscope is calibrated to provide a nominally flat frequency response transfer function from the input of the oscilloscope to the output of the preamplifier.
The compensation circuitry 18 provides resistive and capacitive termination of the cable 16 to minimize reflections and provides a transfer function having a nominally flat frequency response to the measurement test instrument 22. The variable compensation capacitor CC is user adjustable to match the capacitive and resistive divider ratios of the probe over variations in the input capacitance of individual oscilloscope channels. Resistive element RC1 provides resistive cable 16 termination matching into the oscilloscope input at high frequencies (where cable Z0≈155Ω). RC2 in series with variable capacitor CC improves the cable termination into the capacitive load in the oscilloscope.
The tip resistance RT, cable termination resistor RC1 and the input resistance RTS form a voltage divider attenuation network for DC to low frequency input signals. To accommodate a wide frequency range of input signals, the resistive voltage divider attenuation network is compensated using a shunt tip capacitor CT across the tip resistive element RT and a shunt termination capacitor CC and the input capacitor CTS across termination resistive element RTS. To obtain a properly compensated voltage divider, the time constant of the probe tip resistive-capacitive parallel network 12 must equal the time constant of the termination resistive-capacitive parallel network 24 including Ccable and CC.
Properly terminating the resistive cable 16 in its characteristic impedance requires adding a relatively large shunt capacitance CC to the compensation network 18. This is in addition to the bulk cable capacitance CCABLE. For example, the tip resistance RT and capacitance CT for a P2222 10× Passive Probe, manufactured and sold by Tektronix, Inc., Beaverton, Oreg., is selected to give a 10× divide into the oscilloscope's input impedance of 1 MΩ. The minimum tip capacitance CT, neglecting any other parasitic capacitance, is one ninth of the sum of the cable bulk capacitance CCABLE, CC and CTS. The tip capacitance of CT is on the order of 8 pF to 12 pf for the above stated parameters. The input capacitance (which is CT in series with the sum of CCABLE, CC and CTS) is driven by the circuit being monitored and therefore represents a measure of how much the probe loads the circuit.
FIG. 2 illustrates another passive voltage probe and oscilloscope configuration where the preamplifier 28 is configured as a current amplifier. This configuration has the same limitations as the probe and oscilloscope configuration of FIG. 1. The probe has compensation circuitry in the probe compensation box and the oscilloscope has the traditional 1 MΩ resistance in parallel with 10 to 20 pf of capacitance at the oscilloscope input. A major drawback to existing passive voltage probe and oscilloscope configurations is that a substantial portion of the mid-band and high-band frequency signal current at the output of the resistive center conductor signal cable is shunted to ground by the termination capacitor CC. In addition, since the resistive center conductor signal cable is terminated prior to the oscilloscope input, the parasitic capacitance of the input circuitry of the oscilloscope acts as a non-terminated transmission line which shunts additional current to ground.
The probe tip capacitance and the resistive center conductor signal cable affect the overall bandwidth of a traditional passive probe. Further, the probe tip input presents low input impedance to a device under test at high frequencies due to the low capacitive reactance in parallel with the high input resistance. Reducing the probe tip capacitance to increase the capacitive reactance requires adjustment of the other component values of the voltage divider network to maintain a compensated network. Previously, this has been accomplished by increasing the resistance in the probe tip. However, this increases the divider ratio of the network with a resulting increase in the attenuation of signal applied to the probe. The decreased signal input to the oscilloscope may be compensated for by increasing the gain of the oscilloscope input circuits which results in an increase in the noise on the signal reducing the overall signal-to-noise ratio of the instrument.
A special type of passive probe exists that provides a relatively high impedance and attenuation into a 50 ohm input oscilloscope. The Z0 probe has a relatively low input resistance, 5 kilo ohms or less, coupled to a 50 ohm lossless coaxial cable. The capacitance at the probe tip is generally less than 1 pf produced by the parasitic capacitance of the probe head. In a specific embodiment, the probe tip resistance is 450 ohm coupled via the 50 ohm lossless coaxial cable to the 50 ohm input of the oscilloscope, which produces a 10× passive voltage divider network. The voltage input to this probe is limited as compared to the traditional passive probe due to the size of the input resistor. Also, the low input resistance can cause excessive loading to DC signals.
U.S. Pat. No. 6,483,284, shown in FIG. 3, teaches a wideband probe using pole-zero cancellation. A parallel probe tip network of resistor Rtip and capacitor Ctip in series with resistor Rtab and capacitor Ctab detects a signal from a device under test and couples the signal to a compensation network via a near lossless coaxial cable 40. The capacitor Ctab represents the capacitance in the tip circuit, such as a trace on a circuit board, a coaxial cable or the like. A cable termination resistor Re is connected in series between the cable 40 and an inverting input terminal of an operational amplifier 42. The non-inverting input is coupled to a common ground. Connected between the input terminal and the output terminal of the operational amplifier 42 is a parallel combination of a resistor Rfb and a capacitor Cfb with resistor RPk in series with Cfb. The parallel tip resistor Rfp and capacitor Ctip create a zero and the combination of resistor Rtab and capacitor Ctab create a pole. A pole is created by resistor Rfb and capacitor Cf, in the compensation network and a zero is created by resistor RPk and capacitor Cfb. The zero and pole created in the probe tip network are cancelled by the pole and zero in the compensation network. The output of the compensation network is coupled to an end user device, such as an oscilloscope or the like. The teaching states that the time constants of the two RC networks must be equal so that the zeros and poles balance out and the probe has a constant gain. Further, the operational amplifier 42 is part of the wideband probe circuitry and not part of the end user device.